This invention relates generally to a drill for forming holes in metal workpieces and, more particularly, to a drill having cutting inserts which may be easily removed and replaced after the cutting edges become worn.
A drill with a pair of indexable cutting inserts is disclosed in Shallenberger, Jr. U.S. Pat. No. 3,963,365. In that drill, one insert is an outer insert which removes metal from the peripheral wall of the hole and inwardly toward the center of the hole in a cutting arc whose width is equal to one-half the radius of the hole. The other insert is an inner insert which removes metal from the center of the hole and outwardly toward the peripheral wall of the hole in a cutting arc of substantially equal width.
While drills of the type disclosed in the Shallenberger, Jr. patent have enjoyed significant commercial success, the axial feed rate of such a drill is relatively slow due to the fact that a full revolution of the drill is required to cut the full circumference of the hole.
Negishi et al U.S. Pat. No. 4,373,839 discloses a drill which may be fed axially at a rate approximately twice that of the Shallenberger, Jr. drill. This is because the two cutting blades of the Negishi et al drill are positioned so as to cut the entire circumference of the hole during each one-half revolution of the drill. A small diameter core is left between the two blades at the center of the hole but the core is twisted off as the depth of penetration of the drill increases.
Certain embodiments of the drill disclosed in the Negishi et al patent utilize indexable cutting inserts having multiple cutting edges which may be alternately used when a given edge becomes worn. To the best of applicant's knowledge, drills of this type with indexable cutting inserts have never been marketed commercially. Inserts of the type disclosed in the patent are located so close together along their sides that the portion of the drill body between the inserts cannot be made sufficiently strong to withstand the heavy cutting forces which are imposed on the body by way of the inserts during high speed drilling.